October 17, 2011

Top: Visualization of nanoscale disruptions in electron interactions in a Kondo-hole doped heavy-fermion compound. The black-and-white inset shows directly how oscillations in electron behavior are centered on the Thorium impurities, "rippling" outward like disturbances caused by drops of water on a still pond. The rippling oscillations in electron energy are shown in more detail in the close-up view (bottom), where the bands of different shades of blue represent the distance between the ripples.

(PhysOrg.com) -- It's a basic technique learned early, maybe even before kindergarten: Pulling things apart - from toy cars to complicated electronic materials - can reveal a lot about how they work. "That's one way physicists study the things that they love; they do it by destroying them," said Séamus Davis, a physicist at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory and the J.G. White Distinguished Professor of Physical Sciences at Cornell University.

Davis and colleagues recently turned this destructive approach - and a sophisticated tool for "seeing" the effects - on a material they've been studying for its own intrinsic beauty, and for the clues it may offer about superconductivity, the ability of some materials to carry electric current with no resistance. The findings, published in the Proceedings of the National Academy of Sciences the week of October 17, 2011, reveal how substituting just a few atoms can cause widespread disruption of the delicate interactions that give the material its unique properties, including superconductivity.

The material, a compound of uranium, ruthenium, and silicon, is known as a "heavy-fermion" system. "It's a system where the electrons zooming through the material stop periodically to interact with electrons localized on the uranium atoms that make up the lattice, or framework of the crystal," Davis said. These stop-and-go magnetic interactions slow down the electrons, making them appear as if they've taken on extra mass, but also contribute to the material's superconductivity.

In 2010*, Davis and a group of collaborators visualized these heavy fermions for the first time using a technique developed by Davis, known as spectroscopic imaging scanning tunneling microscopy (SI-STM), which measures the wavelength of electrons of the material in relation to their energy.

The idea of the present study was to "destroy" the heavy fermion system by substituting thorium for some of the uranium atoms. Thorium, unlike uranium, is non-magnetic, so in theory, the electrons should be able to move freely around the thorium atoms, instead of stopping for the brief magnetic encounters they have at each uranium atom. These areas where the electrons should flow freely are known as "Kondo holes," named for the physicist who first described the scattering of conductive electrons due to magnetic impurities.

Free-flowing electrons might sound like a good thing if you want a material that can carry current with no resistance. But Kondo holes turn out to be quite destructive to superconductivity. By visualizing the behavior of electrons around Kondo holes for the first time, Davis' current research helps to explain why.

"There have been beautiful theories that predict the effects of Kondo holes, but no one knew how to look at the behavior of the electrons, until now," Davis said.

Working with thorium-doped samples made by physicist Graeme Luke at McMaster University in Ontario, Davis' team used SI-STM to visualize the electron behavior.

"First we identified the sites of the thorium atoms in the lattice, then we looked at the quantum mechanical wave functions of the electrons surrounding those sites," Davis said.

The SI-STM measurements bore out many of the theoretical predictions, including the idea proposed just last year by physicist Dirk Morr of the University of Illinois that the electron waves would oscillate wildly around the Kondo holes, like ocean waves hitting a lighthouse.

So, by destroying the heavy fermions - which must pair up for the material to act as a superconductor - the Kondo holes disrupt the material's superconductivity.

Davis' visualization technique also reveals how just a few Kondo holes can cause such widespread destruction: "The waves of disturbance surrounding each thorium atom are like the ripples that emanate from raindrops suddenly hitting a still pond on a calm day," he said. "And like those ripples, the electronic disturbances travel out quite a distance, interacting with one another. So it takes a tiny number of these impurities to make a lot of disorder."

What the scientists learn by studying the exotic heavy fermion system may also pertain to the mechanism of other superconductors that can operate at warmer temperatures.

"The interactions in high-temperature superconductors are horribly complicated," Davis said. "But understanding the magnetic mechanism that leads to pairing in heavy fermion superconductors - and how it can so easily be disrupted - may offer clues to how similar magnetic interactions might contribute to superconductivity in other materials."

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4 comments

The heavy fermion systems are counterparts of light fermion systems, like the graphene or superconductors. In graphene the electrons are compressed heavily between atoms, so they're moving collectively via fast waves.

Heavy fermion materials suffer with relative lack of electrons and we can compare them to the porous materials, through which the electrons are moving. The motion of every electrons through such "pores" requires the transport of many electrons in perpendicular direction, which will clear a way for the traveling electron, which generates additional magnetic field. As the result, the motion of charge carrier is connected with much larger inertia, the it would correspond the single electron.

Such behavior is typical for the electrons, which are forced to squeeze through tiny holes between atoms (topological insulators) and/or because the electrons are conducted through f-f orbitals outside of atom axis (heavy fermion metals).

The third mechanims, which increases the effective mass of charge carriers is the relativistic effect, because in heavy fermion materials with high atomic mass the electrons are forced to move AROUND atoms at the distance with relativistic speed while gaining mass. Whereas in light fermion materials the electrons are forced to move with relativistic speed BETWEEN atoms while losing mass.

The dual (i.e. opposite) aspect of electron transport in both types of conductors manifests with the opposite effect of the hole doping to the superconductivity: the doping of light fermion materials (graphene) increases the temperature of superconductive transition, because the holes are increasing the mutual compression of electrons within electron fluid.

Whereas at the case of heavy fermion materials the holes are effectively killing the superconductivity of such materials, because they're releasing the pressure of electrons, which are squeezed inside of their orbitals or "pores".

Good research, but the two analogies offered by Prof. Davis are inapt. The first analogy (the lighthouse) is almost on target. The energy causing the waves comes from another source or sources--other than the lighthouse itself. That part is true to the research. The analogy is not perfect, however, because ocean waves generally come from a particular source or direction, rather than from multiple sources.

The second analogy (raindrops in a pond) bothers me much more. There, the energy source for the waves is the raindrop itself. That's the opposite of the circumstances in the experiment. In the experiment, the thorium reflects oscillations that emerged from other (non-thorium) sources. Wild, incoherent oscillations result, because the thorium is particularly efficient at reflecting. These wild oscillations impair the prospects for the relevant oscillators (electrons) to self-organize. When left to their own devices, they self-organize their oscillations into superconductivity.

So what do we learn from this beautiful blue picture above? Prof. Davis says the reflected lighthouse waves (oscillations)disturb the system. I agree, and I offer a rationale..namely, that superconductivity is due to perfect self-organization among electrons, in Cooper pairs, arising when electrons coordinate through their oscillations, antisynchronously.

Normally, heavy fermions have strongly correlated electron systems. But in this case, the thorium "jams" the signals by reflecting the oscillations of the electrons. These reflected oscillations are spurious signals. Absent thorium, the oscillations of each electron act as true signals to all other nearby electrons. These true quantum signals--the oscillations--are the information and the engine that together drive the normal self-organizing behavior of heavy fermions, including their superconductivity.

It's simple..all based on Art Winfree's general theory of coupled oscillators, circa 1967. See Sync by Steve Strogatz.

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